472 ON PHYSIOLOGICAL LIMITS OF MICBOSCOPIC VISION. 



Lastly, in every eye there exist, either in the vitreous humour, 

 lens, (with its capsule) or cornea, particles which, though not 

 ordinarily seen in looking at objects with the naked eye and with 

 natural daylight^ intervene with the effect of shadows throwing 

 opacities whenever a bright light (such as the microscope image with 

 lamplight) is poured through the pupillary aperture. 



The defects hitherto noticed appertain to every healthy eye, and 

 are inherent in the histologic constitution of the structures forming 

 the optical apparatus of the eye. That they do not, strikingly, 

 impair the excellence of vision, as ordinarily exercised hy the naked 

 eye, under suitable conditions of daylight and favourable position 

 of objects seen, is because their ill effects are partly remedied 

 by other circumstances. But it is just when the natural 

 conditions of vision, accommodation of the eye, and muscular 

 movements directed by the mental perception, are most interfered 

 with and strained, — namely, during observation of objects 

 through the microscope — that every natural defect is exaggerated 

 hy the abnormal conditions of vision thus brought about. 

 The necessity of directing the *' mind's eye " to images formed on 

 parts of the retina outside the central axis of vision {i.e:, the act of 

 looking sidewards,) has been already pointed out in connection with 

 the anatomical disposition of the rods and cones of the retina which 

 controls its distinguishing power for coarser details or for more 

 amplified images of finer details. This action is however but part 

 of the whole scheme on which depends the transmission of a 

 microscope image, when physically defined on the retina, to the 

 sensory centres : namely on the susceptibility of the normal and 

 healthy percipient elements to subtle differences of light and shade, 

 {i.e., maxima and minima of intensity of aether undulations,) either 

 as undecomposed white light, or as decomposed into separate colors^ 

 In the same object the extreme range from positive to negative, both 

 of white light and of colors, as well as differeoces of surface from 

 those of least magnitude to those of relatively large areas, may 

 present themselves. Now even under the most favorable condition^ 

 -of light and shade (daylight illumination,) and of definition (normal 



